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NBC 5's Rob Stafford investigates how easy it is to get an arsenal of weapons in Chicago. (Published Tuesday, July 24, 2012)

Updated at 11:51 AM CDT on Tuesday, Jul 24, 2012

Millions of Americans were mortified and surprised Friday to learn that a man in Colorado had used ammunition and military equipment he purchased from the Internet to carry out a deadly and malicious attack at a movie theater.

Jack Riley of the DEA was not among the surprised.

Gangs: Symbols, Colors & Hand Signs

He's recovered AR-15 assault weapons with thousands of rounds of ammo. Trip wires. Tear gas. Explosives and more during his time with the agency. Everything in accused movie theater shooter James Holmes' alleged arsenal is familiar to Riley -- and to the Chicago gangs he investigates.

“It is the way these organizations, whether they be the Mexicans or the street gangs regulate themselves," Riley said. "Through violence. And the tool of that (violence) clearly is weapons and explosives.”

Signs of NATO

Many of the tools of violence are available online. NBC Chicago found a Web site (one that we will not be linking to) that seems to be the Craigslist.org of the gun world. Everything in the Colorado killer's kit is available: a Glock, a Remmington Rifle, an AR-15, and even a 100 round ammo drum. Each item is available for sale in Chicago.

Holmes allegedly received more than 50 packages in four months containing just these items.

Our investigative team, Unit 5, ordered a gas mask from the Web site on Friday, just after the shootings, and it arrived Monday complete with an instruction manual. Cost: $19.99. Tear gas grenades were available on the same site for as little as $10.50.